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            <div><h4 class="title">
                <a name="asio.overview.core.concurrency_hint"></a><a class="link" href="concurrency_hint.html"
                                                                     title="Concurrency Hints">Concurrency Hints</a>
            </h4></div>
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    <p>
        The <a class="link" href="../../reference/io_context/io_context.html" title="io_context::io_context"><code
            class="computeroutput"><span class="identifier">io_context</span></code> constructor</a> allows
        programs to specify a concurrency hint. This is a suggestion to the <code class="computeroutput"><span
            class="identifier">io_context</span></code> implementation as to the number
        of active threads that should be used for running completion handlers.
    </p>
    <p>
        When the Windows I/O completion port backend is in use, this value is passed
        to <code class="literal">CreateIoCompletionPort</code>.
    </p>
    <p>
        When a reactor-based backend is used, the implementation recognises the
        following special concurrency hint values:
    </p>
    <div class="informaltable">
        <table class="table">
            <colgroup>
                <col>
                <col>
            </colgroup>
            <thead>
            <tr>
                <th>
                    <p>
                        Value
                    </p>
                </th>
                <th>
                    <p>
                        Description
                    </p>
                </th>
            </tr>
            </thead>
            <tbody>
            <tr>
                <td>
                    <p>
                        <code class="computeroutput"><span class="number">1</span></code>
                    </p>
                </td>
                <td>
                    <p>
                        The implementation assumes that the <code class="computeroutput"><span class="identifier">io_context</span></code>
                        will be run from a single thread, and applies several optimisations
                        based on this assumption.
                    </p>
                    <p>
                        For example, when a handler is posted from within another handler,
                        the new handler is added to a fast thread-local queue (with the
                        consequence that the new handler is held back until the currently
                        executing handler finishes).
                    </p>
                    <p>
                        The <code class="computeroutput"><span class="identifier">io_context</span></code>
                        still provides full thread safety, and distinct I/O objects may
                        be used from any thread.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <code class="computeroutput"><span class="identifier">ASIO_CONCURRENCY_HINT_UNSAFE</span></code>
                    </p>
                </td>
                <td>
                    <p>
                        This special concurrency hint disables locking in both the scheduler
                        and reactor I/O. This hint has the following restrictions:
                    </p>
                    <p>
                        — Care must be taken to ensure that all operations on the <code class="computeroutput"><span
                            class="identifier">io_context</span></code> and any of its associated
                        I/O objects (such as sockets and timers) occur in only one thread
                        at a time.
                    </p>
                    <p>
                        — Asynchronous resolve operations fail with <code class="computeroutput"><span
                            class="identifier">operation_not_supported</span></code>.
                    </p>
                    <p>
                        — If a <code class="computeroutput"><span class="identifier">signal_set</span></code>
                        is used with the <code class="computeroutput"><span class="identifier">io_context</span></code>,
                        <code class="computeroutput"><span class="identifier">signal_set</span></code> objects
                        cannot be used with any other io_context in the program.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <code class="computeroutput"><span
                                class="identifier">ASIO_CONCURRENCY_HINT_UNSAFE_IO</span></code>
                    </p>
                </td>
                <td>
                    <p>
                        This special concurrency hint disables locking in the reactor
                        I/O. This hint has the following restrictions:
                    </p>
                    <p>
                        — Care must be taken to ensure that the <span class="emphasis"><em>run functions</em></span>
                        on the <code class="computeroutput"><span class="identifier">io_context</span></code>
                        (i.e. <code class="computeroutput"><span class="identifier">run</span></code>, <code
                            class="computeroutput"><span class="identifier">run_for</span></code>, <code
                            class="computeroutput"><span class="identifier">run_until</span></code>,
                        <code class="computeroutput"><span class="identifier">run_one</span></code>, <code
                            class="computeroutput"><span class="identifier">run_one_for</span></code>, <code
                            class="computeroutput"><span class="identifier">run_one_until</span></code>, <code
                            class="computeroutput"><span class="identifier">poll</span></code>, and <code
                            class="computeroutput"><span class="identifier">poll_one</span></code>),
                        and all operations on the context's associated I/O objects (such
                        as sockets and timers), occur in only one thread at a time.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <code class="computeroutput"><span class="identifier">ASIO_CONCURRENCY_HINT_SAFE</span></code>
                    </p>
                </td>
                <td>
                    <p>
                        The default. The <code class="computeroutput"><span class="identifier">io_context</span></code>
                        provides full thread safety, and distinct I/O objects may be
                        used from any thread.
                    </p>
                </td>
            </tr>
            </tbody>
        </table>
    </div>
    <p>
        The concurrency hint used by default-constructed <code class="computeroutput">io_context</code>
        objects can be overridden at compile time by defining the <code class="computeroutput">ASIO_CONCURRENCY_HINT_DEFAULT</code>
        macro. For example, specifying
    </p>
    <pre class="programlisting">-DASIO_CONCURRENCY_HINT_DEFAULT=1
</pre>
    <p>
        on the compiler command line means that a concurrency hint of <code class="computeroutput">1</code>
        is used for all default-constructed <code class="computeroutput">io_context</code> objects in
        the program. Similarly, the concurrency hint used by <code class="computeroutput">io_context</code>
        objects constructed with <code class="computeroutput">1</code> can be overridden by defining
        <code class="computeroutput">ASIO_CONCURRENCY_HINT_1</code>. For example, passing
    </p>
    <pre class="programlisting">-DASIO_CONCURRENCY_HINT_1=ASIO_CONCURRENCY_HINT_UNSAFE
</pre>
    <p>
        to the compiler will disable thread safety for all of these objects.
    </p>
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